security: fix all Critical and High findings from FLAWS_REPORT audit

- Auth: pessimistic locking on login tokens and refresh token rotation,
  backup code attempt counter, rate limiting verification
- Schema: unique constraints on business numbers, FK relations,
  unsigned/signed alignment, attendance duplicate prevention
- Invoices/PDFs: DOMPurify sanitization, bounded queries in stats
  and alerts, VAT rounding, Puppeteer error handling
- Orders/Offers: transactional parent+child creation, Zod NaN
  refinement, status enums, uniqueness checks
- Projects/Files: path traversal protection, streamed uploads,
  permission guards, query param validation
- Attendance/HR: duplicate checks, ownership validation, GPS
  restrictions, trip distance validation
- Frontend: modal lock reference counting, XSS escaping in print
  HTML, ref mutation fixes, accessibility attributes

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/admin/pages/Offers.tsx

@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { useState, useEffect, useRef } from "react";
 import { useAlert } from "../context/AlertContext";
 import { useAuth } from "../context/AuthContext";
 import { Link, useNavigate } from "react-router-dom";
@@ -63,6 +63,16 @@ export default function Offers() {
     quotation: Quotation | null;
   }>({ show: false, quotation: null });
   const [invalidating, setInvalidating] = useState(false);
+  const blobUrlRef = useRef<string | null>(null);
+
+  useEffect(() => {
+    return () => {
+      if (blobUrlRef.current) {
+        URL.revokeObjectURL(blobUrlRef.current);
+        blobUrlRef.current = null;
+      }
+    };
+  }, []);
   const [duplicating, setDuplicating] = useState<number | null>(null);
   const [pdfLoading, setPdfLoading] = useState<number | null>(null);
   const [creatingOrder, setCreatingOrder] = useState<number | null>(null);
@@ -237,9 +247,11 @@ export default function Offers() {
         return;
       }
       const blob = await response.blob();
-      const url = URL.createObjectURL(blob);
-      if (newWindow) newWindow.location.href = url;
-      setTimeout(() => URL.revokeObjectURL(url), 60000);
+      if (blobUrlRef.current) {
+        URL.revokeObjectURL(blobUrlRef.current);
+      }
+      blobUrlRef.current = URL.createObjectURL(blob);
+      if (newWindow) newWindow.location.href = blobUrlRef.current;
     } catch {
       newWindow?.close();
       alert.error("Chyba připojení");